Hypoplastic left heart syndrome (HLHS) is the most common cause of death from heart disease in the first week of life. There are reports about familial concordance by presumed morphogenetic mechanisms of abnormal embryonic blood flow with phenotypes of varying severity. The risk of having a child with a left heart lesion after a previously affected child may be as high as 5% to 12%. We present case reports from four families in which sustained fetal arrhythmia (three ectopic atrial tachycardias and one severe bradycardia due to excessive ectopic atrial beats) was demonstrated. Within these four families a close relative of the mother (a previous child, a brother, or a nephew) had severe left heart abnormality (three with HLHS and one with severe aortic valve stenosis). The association of sustained fetal arrhythmia of ectopic atrial origin and severe left heart abnormalities could be expected to occur by chance in a very low percentage of cases. We conclude that sustained fetal atrial ectopic arrhythmia is a congenital abnormality and should be considered as a risk factor for inherited congenital heart abnormalities.